Six hundred and sixty serum samples from 135 patients treated with human leukocyte alpha interferon (Hu IFN (Le) (New York Blood Center, Melville, N.Y.) for Juvenile Laryngeal Papillomatosis (J.L.P.) were tested for the presence of neutralizing antibodies to the above-mentioned alpha interferon. Eleven samples were found to contain neutralizing antibodies to alpha interferon from the N.Y.B.C. Ten of these contained neutralizing antibodies to human leukocyte alpha interferon (Alferon; Interferon Sciences, New Brunswick, N.J.) and nine contained neutralizing antibodies to recombinant alpha interferon (Intron A, Schering-Plough, Kenilworth, N.J.). None of the samples were seen to bind to either Alferon or Intron in ELISA assays. Six, however, did bind to Wellferon (Burroughs Wellcome, Beckenham, U.K.) which is a natural lymphoblastoid interferon. Since Wellferon is the starting material used in the purification of interferon components in our laboratory, we intend to determine if the samples which bound to Wellferon will also bind to the purified components.